When forming a film of a photoresist agent or functional film whose dry film thickness is 10 μm or less on a semiconductor silicon wafer or glass substrate, application technologies such as spin coaters or bar coaters, etc. have generally been used. This works well with a semiconductor silicon wafer or glass substrate which is flat, or when the surface to which the resist agent is applied is planar, but when the surface is uneven, and not flat, and application is made with a spin coater, the coating material may fly off when performing the necessary rotation of the coated object. Also, it is difficult to form a film with a spin coater or bar coater, etc., on a spherical object or a cylindrical object, which have shapes that cannot be rotated. Also, if the coated surface is uneven (i.e., the surface has a large aspect ratio) or has depressions or holes, it is not possible to coat the uneven part or the sides and bottoms of depressions or the sides of holes, etc.
Therefore, methods have been studied for forming a film of a coating material using a spray gun, but to obtain a film whose dry thickness is 10 μm or less the particle diameter of the atomized liquid is generally about 10 μm to 20 μm, so there are undulations in the coating film, variation in film thickness, bubbles, etc. are attached, considerable time is required to determine the coating condition settings, and it is difficult to obtain a film thickness with good precision. When spraying is performed using an ordinary air spray gun, the particle diameter of the coating material is generally about 10 μm to 15 μm even if viscosity is reduced to 20 CPS or lower.
In that case, when adhering and accumulating coating particles at the uneven part of a 20 μm stepped area, the particle diameter is large, so the coating material sags at the corner of the recessed part and becomes too thin. If one attempts to make the particle diameter finer, such as 10 μm or smaller, a film cannot be formed unless the atomization air pressure increases to 0.4 MPa or higher and the amount dispensed is reduced. In this case, the atomization air pressure is too strong, and the particles that are 10 μm or smaller adhere to the coated object unevenly, and the coating efficiency falls to 30% or less, and it is not successful as an application device. If the usual dry film thickness is 10 μm in an ordinary flat-surface coating, the film thickness precision of a spray is ±10% or more.
When forming a film as thin as 10 μm or less, an air atomization spray is generally the most inexpensive spray system. There are also spray guns which can perform atomization using an ultrasonic atomization system, but the spray speed is too slow, and in practice adhesion to the coated object is uneven, so they are generally used in humidifiers, etc. Also, in airless spray systems and centrifugal atomization systems the viscosity of the liquid is reduced to 20 CPS or lower to form particles of 10 μm and smaller, but they have the defect that at a location 300 mm or farther from the spray exhaust exit only about 20% of the entire dispensed amount is formed, and in addition they are not suitable for low dispensing amounts in which 30 cc or less is applied to the coated object each minute. Therefore, it is customary to consider a two-fluid spray system using air atomization when forming a film as thin as 10 μm or less. However, as described above, the biggest disadvantage is that the coating efficiency is extremely low—about 20-30%—and it has not been possible to achieve a coating film thickness precision of ±5% or less, as with a spin coater, in the film thickness region of 10 μm or less.
Accordingly, using a spray system known as an air brush, which is a special spray system, has also been considered as an air spray. An air brush is a system which utilizes a small hand-held spray gun that is often used when coating plastic components or small products. Its nozzle aperture is 0.5 mm φ or less, and the needle used in coating material exhaust control has a needle shape. When a coating material adheres to and flows out along the needle-shaped needle, the surrounding compressed air atomizes the coating material by the ejector effect. The dispensed amount can be limited to 5 cc or less each minute, and it is possible to form tiny particles of 10 μm or smaller even when the spray nozzle is as close as about 10 mm, so it is possible to coat the coated object with a high efficiency of 80% or higher because the spray nozzle is close.
On the other hand, a liquid two-stage atomization system like that disclosed in Japanese Patent Document JP 2004-89976A, for example, is known as a system for making tiny particles and applying them using an air spray system. In this atomization method, in the first stage a liquid is atomized by compressed air, and in the second stage swirling air acts on the liquid exhaust flow and additionally promotes atomization, and coating is performed as a swirling exhaust flow.
A spray system using the above-described air brush has difficulty controlling small amounts or very small amounts dispensed. That is, adjusting the amount dispensed is a system in which the needle's stroke is increased or decreased by a manual operation, and so it has the problem that quantitative control and adjustment require considerable skill, and therefore automated coating is difficult. In addition, this spray system has the problem that the applied pattern width is narrow: about 5 mm.
Also, the particle-making/application device disclosed in Japanese Patent Document JP 2004-89976A has the advantage that the atomized pattern is wider than in an air brush system, but it has the problem that performing an adjustment to supply tiny amounts of liquid is difficult.
The present invention seeks to solve the problems of the conventional liquid spray devices described above. Its object is to provide a spray device for a small amount of liquid that forms tiny particles of liquid or molten material at the same level or higher than the ultra-tiny particles formed by ultrasonic atomization or an air brush spray system, and that can easily and reliably adjust the supply of liquid to a desired small amount or a very small amount, and that can perform coating of and adhesion to a coated object efficiently, and that can form a uniform and thin film of a liquid such as a liquid photoresist agent, surface protection film, functional coating agent, etc., on a coated object such as a semiconductor silicon wafer, glass substrate, various transparent members, etc., by spray coating.